In the field of seismic isolation the use of sliding bearings is well known. One known type of sliding bearing is a bearing assembly having upper and lower bearing seats and a load bearing sliding member between the seats, the member being able to slide relative to both seats. Examples of such bearing assemblies are in U.S. Pat. No. 4,320,549; U.S. Pat. No. 5,597,239, U.S. Pat. No. 6,021,992, and U.S. Pat. No. 6,126,136.
In another type of sliding bearing the sliding member is fixed to one or other upper or lower bearing seat. In such an embodiment the sliding member is may be a pillar projecting from the bearing seat to which it is affixed. It is usually the upper seat which is movable relative to the slider member. Examples of this type of sliding bearing are found in U.S. Pat. No. 4,644,714; U.S. Pat. No. 5,867,951; U.S. Pat. No. 6,289,640; the embodiments shown in each of FIGS. 4 to 6 in U.S. Pat. No. 6,021,992; and the embodiments shown in FIGS. 4 and 5 of U.S. Pat. No. 6,126,136.
Some of the above mentioned sliding bearings have a curved bearing seat surface and a corresponding curved surface on the sliding element which provide a form of passive self-centring of the sliding element and the bearing seats. None of either types of sliding bearings mentioned above have elastic self-centring.
“Self-centring” is, for the purposes of this specification, urging the sliding element and the upper and lower bearing seats to remain in or return to substantially symmetrical alignment with the longitudinal axis passing through the upper and lower bearing seats and the sliding element perpendicular to a horizontal plane.
An advantage of elastic self-centring is that it provides a means to control the elastic shear stiffness of the bearing to ensure that the isolated structure has a natural period which exceeds the period of the seismic event or other horizontal forces which the bearing assembly is designed to damp so as to enhance the effectiveness of the seismic isolation.
Another advantage, particularly when the sliding member is movable with respect to both the upper and lower bearing seats, is that a bearing assembly may be constructed of a reduced cross sectional area in comparison with a bearing assembly without elastic self-centring. The sliding member in FIGS. 2, 3 and 7 is at rest at the midpoint between the upper and lower seats.